Late in the third instar larval stage of Drosophila melanogaster, the titer of the steroid hormone ecdysone increases sharply. This increase is blocked in the temperature-sensitive mutant ecd1 after a temperature shift from 20 degrees C to 29 degrees C. The mutant was used to identify the major messenger RNA and encoded protein that is induced in the larval fat bodies by the increase in ecdysone titer. The gene coding for the induced messenger RNA has been cloned in a lambda bacteriophage vector. The ecdysone-induced appearance of a major messenger RNA in late third instar larval fat bodies represents a developmental response to ecdysone that appears to be gene-specific, tissue-specific, and stage-specific, and it has exceptionally favorable features for further molecular studies of the control of gene expression by a steroid hormone. Further studies of the genomic organization of this ecdysone-inducible gene and the mechanism of induction are in progress.